Motor Control

ABSTRACT

A motor control circuit for an electric motor of an electric power assisted steering system of the kind in which a measurement of torque carried by a part of the steering system is used to produce a torque demand signal indicative of a torque to be applied to the steering system by the motor, the control circuit comprising a switching circuit comprising a plurality of electrical switches, and a motor current controller that generates voltage demand signals to be passed to a drive circuit for the switches that in turn generates pulse width modulated switching signals for the switching circuit that cause the switches to selectively connect the phases to a power supply so as to cause current to flow through the phases of the motor. The current controller is responsive to an error signal that represents the difference between a current demand signal and an actual current signal, and a noise reduction circuit adapted to identify an operating condition of the system in which the motor is stationary or rotating at a very low speed, and in the event that the operating condition is identified the noise reduction circuit is adapted to reduce the response of the controller to variations in the error signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage of International Application No.PCT/GB2014/050645 filed Mar. 5, 2014, the disclosures of which areincorporated herein by reference in entirety, and which claimed priorityto Great Britain Patent Application No. 1304159.5 filed Mar. 7, 2013,the disclosures of which are incorporated herein by reference inentirety.

BACKGROUND OF THE INVENTION

This invention relates to improvements in motor control circuits for usein electric power assisted steering systems, and in particular to pulsewidth modulation (PWM) control of multiple phase brushless motors inelectric power assisted steering systems.

Control systems for PWM controlled electric motors, especially DCmotors, generally need to measure the current through the windings orphases of the motor and this can either be done by means of separatecurrent sensors for each of the phases, or by means of a single currentsensor that is placed in the circuit so as to measure the totalinstantaneous current flowing between a D.C. power supply and the bridgecircuit and motor combination. In a single current sensor system, themultiple motor phase currents are derived by offsetting the PWM patternsof the switches which apply the required voltage to each phase, andsampling the current sensor at appropriate points.

The measured currents are typically converted into the stationary d-qframe and then combined with a current demand signal, also in the d-qframe, indicative of the current that is demanded from the motor, toproduce an error signal. The demand current in an electric powerassisted steering system is generated as a function of the torquedemanded from the motor. The torque demand signal is a principally ameasure of the amount of torque the motor should apply to the steeringto help the driver to turn the wheel.

The error signal represents the difference between the current that isdemanded and the actual measured current. The error signal is fed to acontroller which produces a set of voltage demand signals, alsotypically in the d-q frame, representative of the voltage to be appliedto the motor that will best drive the error signal towards zero. The d-qvoltages are then converted into PWM signals for the motor phasesdepending on which PWM strategy is used. The controller therefore actsto vary the PWM phase voltages in order to try to constantly minimisethe magnitude of the error signal thereby ensuring that the motorcurrent is as close as possible to the demanded current.

In a practical system the current controller will comprise a PI or PIDor other type of feedback controller. The role of the current control isto modify the voltages applied to the motor with an aim of keeping theerror signal value as small as possible. The controller forms a closedloop. Around the current controller the torque demand signal is producedby the torque controller, which forms another closed loop with ameasurement or estimate of the torque in the steering as one input.

Motor drive circuits using feedback control and PWM are well known inthe art. For example, WO2006005927, discloses a typical system and theteaching of that document is incorporated herein by reference.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect the invention provides a motor controlcircuit for an electric motor of an electric power assisted steeringsystem of the kind in which a measurement of torque carried by a part ofthe steering system is used to produce a torque demand signal indicativeof a torque to be applied to the steering system by the motor, the drivecircuit comprising:

-   -   a switching circuit comprising a plurality of electrical        switches,    -   a motor current controller that generates voltage demand signals        to be passed to a drive circuit for the switches that in turn        generates pulse width modulated switching signals for the        switching circuit that cause the switches to selectively connect        the phases to a power supply so as to cause current to flow        through the phases of the motor,    -   in which the current controller is responsive to an error signal        that represents the difference between a current demand signal        and an actual current signal, and    -   a noise reduction circuit adapted to identify an operating        condition of the system in which the motor is stationary or        rotating at a very low speed, and in the event that the        operating condition is identified the noise reduction circuit is        adapted to reduce the response of the controller to variations        in the error signal.

By reducing the response of the controller to variations in the errorsignal, such as high frequency variations caused by quantisation errorsin the output from a current sensor, the noise reduction circuit maymake the controller insensitive to changes in the error signal or reducethe magnitude of any change in the output of the controller in responseto a corresponding change in the error signal. In each case, the gain ofthe controller is effectively reduced when the operating condition inwhich the motor is stationary or rotating at very low speed.

The applicant has appreciated that significant noise can be present athigh frequencies in the actual current signal, typically output from acurrent sensor. This may be due to quantisation of the current signalwhere the sensor can only adopt a discrete number of output states, thevalue changing at a rate which depends on how often the sensor output isupdated, or at the frequency of any noise in the sensor itself. If theactual current is somewhere between quantisation states it can oscillaterapidly between the states as the current signal is produced. This noisewill be passed onto the error signal, and if left unchecked will causethe current controller to react because it cannot discriminate betweennoise and genuine changes in error that should be tracked. This noisecan cause unwanted acoustic noise, especially when the motor isstationary—when it is rotating the quantization noise is disguised asthe motor current will be sinusoidally varying.

The noise reduction circuit may be adapted to reduce the response of thecontroller by disabling the current controller so that the output of thecontroller is not responsive to the error signal when the certainoperating condition is met. When not responsive the current controllermay maintain at an output the values that were provided before the noisereduction circuit disable the controller. The controller will no longertrack changes in the error signal.

Alternatively, the noise reduction circuit may include an attenuator andmay be adapted to reduce the sensitivity of the controller to changes inthe error signal by attenuating the error signal that is fed to thecontroller when the certain condition is met. When the error signal isattenuated the magnitude of the noise is reduced, so the reaction of thecurrent controller to the noise is likewise reduced. The currentcontroller will then respond to the attenuated error signal rather thanthe original, unmodified, signal.

In a further alternative, the noise reduction circuit may include afilter and may be adapted to reduce the sensitivity of the controller tochanges in the error signal by passing the error signal through afilter. This may comprise a low pass filter. The current controller willthen respond to the filtered error signal rather than the original,unfiltered, error signal.

The noise reduction circuit may include both an attenuator and a filter,the error signal being passed through the filter and attenuated.

Where the noise reduction circuit comprises an attenuator it may beadapted to attenuate the error signal with a fixed gain or a variablegain.

Where a variable gain is applied the gain may be function of magnitudeof error signal, the magnitude of the attenuation being higher for afirst error magnitude and lower for a second, higher, error magnitude.The attenuation may be varied continuously over a range of error valuesaround zero error and up to a predefined maximum error above which noattenuation is provided.

The noise reduction circuit may attenuate the error signal bymultiplying the error signal by a gain term, or dividing it by anattenuation term, to provide a modified error signal that the currentcontroller reacts to.

The noise reduction circuit may determine that the certain operatingcondition is met in the event that the motor velocity is below athreshold motor velocity.

The noise reduction circuit may, instead of using an absolute threshold,apply hysteresis to a signal indicative of motor velocity and comparethe signal with hysteresis to the threshold to determine if thecondition is met. The use of hysteresis may advantageously stop thenoise reduction being switches on and off too rapidly.

The noise reduction circuit may also determine that the condition is metif a speed of a vehicle to which the steering is fitted is below athreshold level. It may determine that the condition is met by using ameasure of vehicle speed rather than motor velocity, or by using acombination of the two measures.

The current controller may receive a current error signal that isexpressed in the d-q axis frame, and the sensitivity of the currentcontroller to the error signal may be in respect of the q-axis componentonly, or the d-axis component, or both.

The apparatus may be adapted to determine that the motor is rotatingslowly or is stationary by measuring or estimating the speed of themotor. It may therefore include means for receiving a signal indicativeof the speed of the motor.

According to a second aspect the invention provides a method ofcontrolling an electric motor using a current controller of the kindthat generates voltage demand signals to be passed to a drive circuitfor the motor that in turn generates pulse width modulated switchingsignals for a switching circuit that cause the switches to selectivelyconnect the phases of a motor to a power supply so as to cause currentto flow through the phases of the motor, in which the current controlleris responsive to an error signal that represents the difference betweena current demand signal and an actual current signal, the methodcomprising the steps of identifying an operating condition of the systemin which the motor is stationary or rotating at a very low speed, and inthe event that the operating condition is identified reducing theresponse of the controller to variations in the error signal.

The method may comprise reducing the response of the controller tovariations in the error signal by at least one of the following:

-   -   Disabling the current controller;    -   Attenuating the error signal and causing the current controller        to respond to the attenuated error signal; and    -   Filtering the error signal.

The method may comprise attenuating the error signal by a fixed or avariable amount.

The method may comprise attenuating the error signal by a variableamount as a function of the magnitude of the error signal. The errorsignal may be attenuated by a higher amount as the magnitude of theerror signal becomes closer to zero.

The variable amount may be varied between a discrete number of levels,for example two or three levels, or may be continuously varied. It may,for example, vary linearly as a function of error magnitude.

According to a third aspect the invention provides an electric powerassisted steering system comprising a steering mechanism that connects asteering wheel to a road wheel, a torque sensor that produces a torquesignal indicative of the torque carried by a part of the steeringmechanism, an electric motor that is connected to the steering mechanismso that torque produced by the motor is transferred to the steeringmechanism, a means for generating a torque demand signal indicative of atorque to be applied to the steering system by the motor that is afunction of the torque measured by the torque sensor, and a motorcontrol circuit that is responsive to a signal dependent on the torquedemand signal, the motor control circuit being arranged in accordancewith the first aspect of the invention.

The motor may comprise a three phase brushless DC motor.

Other advantages of this invention will become apparent to those skilledin the art from the following detailed description of the preferredembodiments, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a part of an embodiment of a motorcontrol circuit of the present invention;

FIG. 2 is an illustration of the effect of noise in the current feedbacksignal in an open loop control mode;

FIG. 3 is an illustration of the effect of the same noise in a closedloop control mode of the current controller;

FIG. 4 is an illustration showing the current measurement valuesobtained when the current in a three phase motor is varyingsinusoidally;

FIG. 5 shows a representative variation in current error signal valueover time alongside a plot showing the noise reduction circuit switchingnoise reduction on and off over the same timescale as the magnitude ofthe error signal value crosses a threshold, hysteresis being applied;

FIG. 6 is block diagram of a suitable structure of the noise reductioncircuit 40 when switched on and attenuating the error signal;

FIG. 7 is a corresponding block diagram showing the same noise reductioncircuit 40 when switched off and applying no attenuation;

FIG. 8 is a plot of absolute current error against gain for the noisereduction circuit with use of hysteresis;

FIG. 9 is a plot of absolute current error signal value against gain forthe noise reduction circuit;

FIG. 10 shows a motor electrical circuit and switching circuit for athree phase DC brushless permanent magnet motor; and

FIG. 11 is an overview of a complete electric power assisted steeringsystem with a motor and motor control circuit in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 10 a three phase brushless motor 1 comprises threemotor windings 2, 4, 6, generally designated as phases A, B and C. Thephases are connected in a star network but could be connected in anotherarrangement, such as a delta topology. One end of each coil is connectedto a respective terminal. The other ends of the coils are connectedtogether to form the star centre 7. The free ends are connected to aswitching circuit arranged as an H-bridge.

The switching circuit comprises a three phase bridge 8, one for eachphase of the motor. Each arm 10, 12, 14 of the bridge comprises a pairof switches in the form of a top transistor 16 and a bottom transistor18 connected in series between a supply rail 20 and ground line 22. Themotor windings 2, 4, 6 are each tapped off from between a respectivecomplementary pair of transistors 16, 18.

The transistors 16, 18 are turned on and off in a controlled manner by amotor controller 21, which is shown in detail in FIG. 3 of the drawings,to provide pulse width modulation of the potential applied to each ofthe phase windings, thereby to control the potential difference appliedacross each of the windings 2, 4, 6 and hence also the current flowingthrough the windings. This in turn controls the strength and orientationof the magnetic field produced by the windings, which sets the torqueproduced by the motor.

A current measuring device in the form of a resistor 24 is provided inthe ground line 22 between the motor 1 and ground so that the controller21 can measure the total current flowing through all of the windings 2,4, 6. In order to measure the current in each of the windings the totalcurrent has to be sampled at precise instances within the PWM periodwhere the voltage applied to each terminal of the winding (and hence theconduction state of a particular phase) is known. If preferred aseparate current sensor could be provided for each phase.

The controller in this example uses a Space Vector Modulation (SVM)algorithm although any modulation technique can equally be used withinthe scope of the present invention and this should not be construed aslimiting.

Each winding 2, 4, 6 in a three phase system can only be connected toeither the supply rail 20 or the ground line 22 and there are thereforeeight possible states of the switches of the control circuit. Using 1 torepresent one of the phases being at positive voltage and 0 to representa phase connected to ground, state 1 can be represented as [100]indicating phase A at 1, phase B at 0 and phase C at 0, State 2 isrepresented as [110], state 3 as [010], state 4 as [011], state 5 as[001], state 6 as [101], state 0 as [000] and state 7 as [111]. Each ofstates 1 to 6 is a conducting state in which current flows through allof the windings 2, 4, 6, flowing in one direction through one of themand in the other direction through the other two. State 0 is a zero voltstate in which all of the windings are connected to ground and state 7is a zero volt state in which all the windings are connected to thesupply rail.

During normal operation when the switching circuit is being controlledby the controller 21 to produce pulse width modulation, each of thephases 2,4,6 will normally be turned on and off once in each PWM period.The relative lengths of time that are taken up in each state willdetermine the magnitude and direction of the magnetic field produced ineach winding, and hence the magnitude and direction of the total torqueapplied to the rotor.

The motor control circuit of FIG. 1 and FIG. 11 can be used in manyapplications, and in this embodiment forms part of an electric powersteering system. The steering system includes a torque sensor 26 thatmeasures the torque in a steering column, and feeds this to a torquecontroller. The torque controller calculates a torque demand signal thatis indicative of the torque that is to be produced by the motor. This isfed through into the current controller, which converts the torquedemand into a current demand according to known characteristics of themotor. The controller then causes the motor to operate and provide thedemanded torque. This torque is applied by the motor to the steeringsystem, making it easier for a driver to turn the steering wheel. Thesystem therefore has two closed loops—the torque controller loop and thecurrent controller loop.

The motor control circuit takes the demanded torque and a measure of themotor rotor position w and generates a current demand signal in thefixed d-q frame. This signal is then fed into a subtractor 32 of thecurrent controller along with a measure of the actual current flowing inthe motor, expressed in the d-q frame and derived from the output of thecurrent sensor 24. The output of the subtractor is an error signalindicative of the difference between the demanded d-q current and theactual measured d-q current.

The error signal is then passed through a noise reduction circuit 30 andfed to the input of a proportional-integral (PI) controller stage 34that converts the current error signal which is in the d-q frame into avoltage signal in the d-q frame, and this is then converted into threephase voltages UVW by a dq-UVW converter 36. This is then converted intothe required PWM voltage signals for each phase using the chosen SVMmodulation technique by a PWM converter 38. The controller constantlyattempts to drive the error signal to zero, which ensures that thecurrent in the motor phases matches the demanded current.

The current controller within the EPS system requires a high bandwidthto achieve the desired steering performance. With a high bandwidthcontroller any noise that falls within the bandwidth of the controllerwill be reacted to—the controller does not know the difference betweenthe true signal and the measurement noise.

There are multiple sources of noise and for the current controller thereare two main sources:

-   -   1. Demand signal    -   2. Feedback signal

The reaction of the controller to this noise is to attempt to cancel itout (in the feedback) or track it (in the demand) with the result thatthe control action applied to the motor contains the noise component.This injected noise can cause unwanted NVH effects, either throughhaptic or acoustic effects.

FIG. 2 shows an open loop system where the controller is disabled and aconstant voltage is applied to the motor. The current has no noise andis measured by the ECU; the measurement signal has noise added to it.Closing the loop and enabling the controller (FIG. 3); the controllersees the measurement noise which appears an error between the demand andthe measured signal. The controller, attempting to remove the noise,applies voltage demands to the motor containing this noise with resultthat noise is present in the real current.

Typically this noise is only heard when the motor is stationary orrotating slowly, where the phase currents are almost constant, and willmanifest as a rumble noise. When the motor is rotating the phasecurrents are sinusoidal and therefore the noise disappears as thecurrents are varying.

FIG. 5 shows typical sinusoidal currents over one electrical revolution.As the motor rotates the currents change; when the motor is stationarythe currents are static, an example high lighted at 1rad. Rotating thenoise and quantisation levels of the current measurements are hidden asthe currents sweep through the various levels. Stationary the smallchanges in ADC quantisation can be detected, small changes in currenttranslate into small changes in torque which the driver is sensitive to.

The applicant has appreciated that one of the main contributors to thenoise in the feedback current is the resolution of the currentmeasurement. Typically for an electric power assisted steering (EPS)application higher power motors are required which require large currentlevels. As the current range increases the resolution of the measurementis degraded, e.g. with a 10 bit ADC the measurement is quantised to 1024levels. For an EPS system which can measure current up to ±150 A thismeans that the resolution is about 0.3 A. When the motor is stationaryand the phase currents are constant this quantisation is an issue andthe controller jumps between the levels, e.g. for a 10 A demand themeasured current will be 9.7 A, 10 A and 10.3 A. It is these stepschanges that the control reacts to.

In this embodiment, a noise reduction circuit is provided that is onlyactivated under specific circumstances when it is known that rumble canbe present. At all other times, for full dynamic steering performancethe noise reduction circuit is disabled. The algorithm only operateswhen particular conditions are fulfilled, signified by rumble conditionsmet, although this is not directly relevant to the patent application.The conditions are:

-   -   Vehicle speed is below a threshold specified by rumble vehicle        speed enable.    -   Motor speed is below a threshold specified by rumble motor speed        enable.

Hysteresis is optionally applied to both of these signals. An exampleimplementation could be:

IF (ABS(motor mechanical velocity) >= (RUMBLE MOTOR SPEED ENABLE +RUMBLE MOTOR SPEED HYSTERESIS)) OR (vehicle speed >= (RUMBLE MOTOR SPEEDENABLE + RUMBLEX MOTOR SPEED HYSTERESIS))    rumble conditions met =FALSE ELSE    IF (ABS(motor mechanical velocity) < RUMBLE MOTOR SPEEDENABLE) AND               (vehicle speed < RUMBLE VEHICLE              SPEED ENABLE)       rumble conditions met = TRUE    ENDIFENDIF

The noise reduction circuit operates on the controller error (e DQ inFIG. 1), the signal that drives the current controller rather than theseparate demand and actual feedback current signals.

d axis current error=d axis current demand−d axis current

q axis current error=q axis current demand−q axis current

The rumble issue only occurs when the motor is stationary or rotatingvery slowly, which for the current controller is almost a steady statecondition. This is used to activate the rumble compensation, i.e. whenthe error is almost zero then compensation can be activated, otherwisethe control must react with full bandwidth to remove the error.Hysteresis can be applied to the enable threshold to minimise cycling inand of the condition as shown in FIG. 5.

The compensation can be activated as:

IF ( (ABS(q axis current error) >= rumble motor control current disable)OR              (rumble conditions met == FALSE) )      rumble motorcontrol current active = FALSE ELSE    IF ( (abs_q_axis_error < RUMBLEMOTOR CONTROL CURRENT ENABLE) AND              (rumble conditions met ==TRUE) AND              (rumble motor control current active ==             FALSE) )       rumble motor control current active = TRUE   ENDIF ENDIF

When rumble motor control current active is TRUE there are a number ofpossible options:

-   -   1. Disable the current controller    -   2. Attenuate the error signal by a fixed gain    -   3. Attenuate the error signal by a variable gain    -   4. Filter the error signal

A combination of the above may be suitable, depending on theapplication, e.g. the filtered error signal may be used to drive thevariable gain approach, e.g. as shown in 6 and FIG. 7 (active in FIG. 6and inactive in FIG. 7).

The benefit of the provision of the novel noise reduction circuit withinthe current controller is that the compensation only becomes active whenthe current controller has reached the target demanded therefore thefull bandwidth is present for dynamic situations. As shown a bypassswitch 40 is opened and closed to activate and deactivate the noisereduction circuit.

One option is to disable the current controller when rumble currentcontrol active is TRUE. By disabling the current controller the lastvoltage output by the controller is held, maintaining the set pointuntil the absolute error exceeds rumble motor control current disable.Although this is the ultimate solution (the controller is frozen so nosignal is propagated) in practice this can have an adverse effect on thesteer-feel although for some applications where steer-feel is not anissue this may be an option.

Another option is attenuate the error signal, i.e. when rumble motorcontrol current active is TRUE reduce the error terms (D and Q) byRUMBLE PI ERROR GAIN which is a value between 0 and 1.0. A RUMBLE PIERROR GAIN of 0 means that the error term is zero, effectively freezingthe current controller. A RUMBLE PI ERROR GAIN of 1.0 has no effect andthe PI controller operates as normal. Selection of a value in betweenreduces the error term and reduces the response of the controller with asubsequent reduction in rumble as shown in FIG. 8.

d axis current error=d axis current error×RUMBLE PI ERROR GAIN

q axis current error=q axis current error×RUMBLE PI ERROR GAIN

Note that a RUMBLE PI ERROR GAIN of zero is the same as disabling thecurrent controller.

Another option within the scope of the present invention is to vary thelevel of attenuation depending on the level of error. As the error termapproaches zero it is attenuated more strongly as illustrated in FIG. 9which is a plot of error magnitude against error gain.

Another option is to filter the error signal, initialising the filter atactivation with the last unfiltered measurement to provide a smoothertransition into the filtered case. Unlike with the original strategy thefilter is only active when the target current has been met therefore thedestabilising effect of the filter lag is not an issue.

In accordance with the provisions of the patent statutes, the principleand mode of operation of this invention have been explained andillustrated in its preferred embodiments. However, it must be understoodthat this invention may be practiced otherwise than as specificallyexplained and illustrated without departing from its spirit or scope.

1. A motor control circuit for an electric motor of an electric power assisted steering system of the kind in which a measurement of torque carried by a part of the steering system is used to produce a torque demand signal indicative of a torque to be applied to the steering system by the motor, the motor control circuit comprising: a switching circuit comprising a plurality of electrical switches, a motor current controller that generates voltage demand signals to be passed to a drive circuit for the switches that in turn generates pulse width modulated switching signals for the switching circuit that cause the switches to selectively connect the phases to a power supply so as to cause current to flow through the phases of the motor, in which the motor current controller is responsive to an error signal that represents the difference between a current demand signal and an actual current signal, and a noise reduction circuit adapted to identify an operating condition of the system in which the motor is stationary or rotating at a very low speed, and in the event that the operating condition is identified the noise reduction circuit is adapted to reduce a response of the motor current controller to variations in the error signal.
 2. The motor control circuit according to claim 1 in which the noise reduction circuit is adapted to reduce the response of the motor current controller by disabling the motor current controller so that an output of the motor current controller is not responsive to the error signal when the certain operating condition is met.
 3. The motor control circuit according to claim 1 in which the noise reduction circuit includes a filter and is adapted to reduce the sensitivity of the motor current controller to changes in the error signal by passing the error signal through a filter, the motor current controller reacting to the filtered error signal output from the filter.
 4. The motor control circuit according to claim 1 in which the noise reduction circuit includes an attenuator and is adapted to reduce the sensitivity of the motor current controller to changes in the error signal by attenuating the error signal, or filtered error signal, that is fed to the motor current controller when the certain condition is met.
 5. The motor control circuit according to claim 4 in which the noise reduction circuit comprises an attenuator adapted to attenuate the error signal with a fixed gain or a variable gain.
 6. The motor control circuit according to claim 5 where the variable gain is a function of a magnitude of the error signal, the magnitude of the attenuation being higher for a first error magnitude and lower for a second, higher, error magnitude.
 7. The motor control circuit according to claim 1 in which the noise reduction circuit receives a signal indicative of a velocity of the motor and is adapted to determine that the certain operating condition is met in the event that the motor velocity is below a threshold motor velocity.
 8. The motor control circuit according to claim 7 in which the noise reduction circuit applies hysteresis to a signal indicative of motor velocity and compares the signal with hysteresis applied to the threshold motor velocity to determine if the certain operating condition is met.
 9. The motor control circuit according to claim 1 in which the noise reduction circuit is adapted to receive a signal indicative of a speed of a vehicle to which the steering system is fitted, and in which the noise reduction circuit determines that the certain operating condition is met if the speed of the vehicle is below a threshold level.
 10. The motor control circuit according to claim 1 in which the current controller receives a current error signal that is expressed in the d-q axis frame, and a sensitivity of the current controller to the error signal is in respect of a q-axis component only, or the a d-axis component, or both.
 11. A method of controlling an electric motor using a current controller of the kind that generates voltage demand signals to be passed to a drive circuit for the motor that in turn generates pulse width modulated switching signals for a switching circuit that cause the switches to selectively connect the phases of a motor to a power supply so as to cause current to flow through the phases of the motor, in which the current controller is responsive to an error signal that represents the a difference between a current demand signal and an actual current signal, the method comprising the steps of identifying an operating condition of a system in which the motor is stationary or rotating at a very low speed, and in the event that the operating condition is identified reducing a response of the current controller to variations in the error signal.
 12. The method according to claim 11 that further comprises reducing the response of the current controller to variations in the error signal by performing at least one of the following: disabling the current controller; attenuating the error signal and causing the current controller to respond to the attenuated error signal; and filtering the error signal.
 13. The method according to claim 12 that further comprises attenuating the error signal by a fixed or a variable amount.
 14. The method according to claim 13 that comprises attenuating the error signal by a variable amount as a function of a magnitude of the error signal.
 15. (canceled)
 16. (canceled)
 17. (canceled) 